Archives For Reigate

A cold snap looks more likely next week from around 13 January. Nothing extreme, just a long-overdue “normal” wintry feel is on the cards.

12Z gefs and ecm ensembles show more distinct dip in temps

ECMWF upper air goes cold

Temperatures are due to take a dip below the seasonal norm. It’s still a way off so details will change but here’s the current idea moving ahead.

The current run of wet Atlantic westerlies (above) that has brought flooding to many parts of the country, is due to weaken as pressure rises over the Atlantic and further north over the Pole. As the persistent Atlantic LOW pressure gradually moves East this week it will bring more rain across the UK. As it moves further east over the weekend it is forecast to draw down cooler northerly winds from the Arctic next week, at least for a while. (see below). (update: “Atlantic block” noted on chart below is probably overstating it a bit … as HIGH is likely to give way fairly promptly)

Arctic Oscillation goes negative january 2016

The cause of this Arctic outbreak is indicated by the Arctic Oscillation (AO) going negative. The AO is a measure of air pressure over the Pole relative to mid-latitudes. It has been positive for most of the autumn and winter so far and this usually means a strong jetstream and mild wet westerlies for the UK.

12Z GFS ENS maintains the theme for a more pronounced Greenland/Polar block and here’s some further info. pic.twitter.com/wpXJxM6rLD

When the AO goes negative it indicates building pressure over the Pole and more likelihood of Arctic air “leaking” into mid-latitudes. The chart below shows the 500mb mean heights for 8-10 days time. Spot the anomalously high pressure over the Pole relative to the mid-latitudes.

8-10 day mean 500mb heights (ecm and gfs models)

The negative AO has been predicted by many expert long range forecasters for a long time partly because of a lack of sea ice in the Kara Sea (part of the Arctic Ocean near Siberia). This is a long-term indicator for potential pressure rises in this region.

In addition, the recent enormous pump of warm air, courtesy of the Storm Frank, will have encouraged tropospheric height rises over the Pole. The result is an inflating balloon of relatively cold air waiting to pop into the populated mid-latitudes!

surface temperature anomalies Jan 14 2016

As well as a push of cooler Arctic air, pressure is also due to rise over the UK. With increased pressure we can thankfully expect a drier period.

wintry pressure rise

A classic winter high might be expected to bring dry, bright clear days with cold nights with views of the stars. On the other hand, frost, fog and icy conditions might also be expected too. In the SE huge dumps of snow look most unlikely next week from current model runs but lows can move south in the Arctic flow and cause unexpected events. The duration of the cold snap doesn’t look long** as the Atlantic HIGH is swept away by more westerlies. However, this is uncertain so stay tuned for more regular updates on twitter. Of course, seek professional forecasts for decision making purposes.

The synoptic situation above shows the remarkable contrast building up around Europe this week. While much of the UK still sits in a mild SW flow brought in by a powerful SW jetstream, a HIGH pressure building over Scandinavia is set to drag in Siberian easterly winds to Eastern and SE Europe which is due to get much colder.

early December 2015 warm

Winter arrives New Year in E Europe

Whilst the jetstream is forecast to sink a little south of the UK by early January, which is the colder side, it is not certain whether any proper cold will reach the UK just yet. The UK looks increasingly sandwiched between bitterly cold easterly air and cool Atlantic NW air. A stormy and wet set-up is likely with temperature contrasts like these, but will it snow?

The ECMWF is certainly keen on something cold as pressure falls across the UK and Europe and allows the chance of LOWS ingesting nearby cold continental air. This could mean snowy weather for the North, especially on hills, in early January, for example.

However, for the SE Tmins stay above freezing on the chart below for nearby Gatwick, Surrey, and note the absence of snowfall, at least for now.

While the ECM shows the potential for cold UK conditions by early January, the GFS maintains a more broadly westerly Atlantic flow with temperatures falling to average.

ensembles agree on cooler average temps, at least

rainfall totals bump up

Both show a cooler regime and more persistent rainfall for us in the SE. A fall in pressure means LOWS will also make more direct passage across the UK during early January.

The jetstream is shown to move south of the UK over the next few weeks. This will bring colder air across the country and lower pressure.

powerful jet blasting north of the UK into Arctic

sinks south by early January

The terrible flooding in the North and West of England and Wales contrasts with the drier than normal December down here in the SE, running at about 70% of normal rainfall so far this month. Unfortunately, a significant Atlantic storm, named Frank, is bombing-out right now in the Atlantic and is set to bring gales and more heavy rain to the NW, albeit not due to track directly across the UK.

Gales mainly out west

Forecast impacts of Storm Frank (typo:SW not SE!)

Storm Frank will stay mostly in the Atlantic and arrive in Iceland on Wednesday night. It is unlikely to impact us much here in the south east other than some blustery and wet weather especially through Wednesday pm/evening as the cold front passes over.

fronts out west move slowly east

rainfall totals from Storm Frank

Polar Maritime air behind this front will bring a cooler average feel by Thursday. Further wet and windy weather is likely later in the week and into New Year. The arrival of this NW/westerly air probably spells an end to the extraordinarily warm long-fetch south westerlies that have made December 2015 by far the warmest on record. Whilst there will be warm sectors passing through the persistent warmth is less likely through January.

CET warmest December on record 2015

earliest flowering Rhododenrons

The wider atmospheric conditions hold more interesting clues than models as to which way the weather could proceed through January. A more powerful than usual stratospheric vortex has built around the Pole this December. The extra-powerful vortex has possibly been caused by the excessive heat injected into the global atmosphere by the mega-El Nino: increasing the temperature gradient between mid-latitudes and the Pole and thereby increasing the strength of polar vortex as temperatures in the Polar stratosphere have fallen in the polar night. Much catastrophic weather has been blamed on the El Nino “bar fire” burning across the Pacific. Whilst Texas tornadoes and Pacific hurricanes are more likely to be directly linked to ENSO, UK flooding and weather has only tenuous links. El Nino has now reached a peak but will continue to output through Spring until likely turning neutral and even reaching an opposite cool La Nina state by next winter. (more on El Nino impact on UK weather here )

Mega El Nino December 2015

Late December powerful stratospheric vortex

Jan 7 vortex squeezed, displaced but not destroyed

The powerful stratospheric vortex has “sealed in” cold air into the Pole throughout December (Globe a below). However, the vortex has been taking a hammering from perturbations from the troposphere known as vertical waves (Globe b). If sufficiently powerful, these waves can lead to sudden warming in the stratosphere which can distort, split or even destroy the vortex, allowing cold Polar air to “escape” into mid-latitudes (Globe c).

stratospheric warming December 2015

stages of stratospheric break up

December warming at 10hPa, but not enough

Stratospheric experts differ somewhat in their forecasts but, overall, the feeling is that something is afoot high up which could erupt into a full break up of the vortex by Mid-January, something called a sudden stratospheric warming. This what a SSW looks like in 3D.

A Sudden Stratospheric Warming could then mean, depending on how surface pressure patterns pan out, that some sustained proper cold arrives a few weeks later around late January into February. This is due to pressure rising over the Poles as a result of warming in the Stratosphere. The increased pressure over the Poles further strengthens the negative Arctic Oscillation giving greater risk of cold outbreaks across mid-latitudes and snowy weather. Even without a major warming, the super-powerful zonal westerly winds in the stratosphere appear to be abating (chart below: top arrows) due to the perturbation from the troposphere which is forecast to continue. Whilst lower down the tropospheric jetstream nudges south of our latitude (lower black arrow) as anomalous or neutral winds expand out from 60N (red circles). This means an increased possibility of cold leaks from the Pole reaching our latitude above 50N.

Unfortunately, at the moment, none of this is catastrophic for the stratospheric vortex because, to date, the waves and warming have been insufficient to knock this King Vortex off his perch. Nevertheless, El Nino years see a greater chance of SSWs and, as the westerly QBO weakens, it seems odds-on for such a full sudden stratospheric warming event.

deep Atlantic trough squares up to Scandi high

500mb action suggests changing pattern

Meanwhile, action nearer the surface in the troposphere (up to 10km) has been even more interesting with changes taking place that may render any stratospheric influence less relevant at least in the medium term. Notably, the Arctic and North Atlantic Oscillations are both trending negative. This means pressure is building in the north relative to the mid-latitudes and could allow more polar air to push out into Europe. A cold outbreak is also possible in the USA.

Arctic Oscillation goes negative

North Atlantic Oscillation goes negative

Until now, December has been dominated by a positive Arctic Oscillation and positive North Atlantic Oscillation, hence the mild conditions. A change to negative AO and NAO is therefore a significant indicator of cooler weather ahead.

warming Pole negative AO

The chart above shows mean anomaly 2m temperatures for the 5 days 08-13 Jan 2016. Note the extraordinary warmth over the Pole which helps to build pressure. Note also the cold pool in the Atlantic, associated with cooler sea surface temperatures residing there. This will be significant because NW polar maritime winds will be cooler than usual and could bring more snow to the NW and especially upland UK, even if we fail to get any truly bitter easterlies. Spot the cold lurking in the N/east of Europe and Russia waiting to pounce should we get a LOW sliding east through the Channel, for example! Finally, the MJO is an important influence on winter outcomes worth exploring.

The MJO or MOJO or Madden Julien Oscillation is a measure of convective activity which starts in the Indian Ocean and migrates east as a series of thunderstorms across to the Pacific and thence around the planet in a 30-60 day cycle. Here is a video about how the MJO impacts Australia, but it also impacts weather elsewhere.

The intensity and state of the MJO connects with global weather and correlates to known pressure patterns in the northern hemisphere. We have just left MJO Phase 5. The expected mean pressure pattern associated with MJO Phase 5 (in ENSO positive phase) correlated pretty well with the pattern that turned out. See charts below.

expected MJO mean 500mb pattern

actual Deecember 2015 MJO Phase 5

The MJO is now going through a nicely organised phase. This could mean that the correlation with real weather patterns continues into the weeks ahead. Here below is the expected 500mb pressure pattern for Phase 6, which we are just entering and then Phase 7 and 8 which are due in early January. Note the strong westerlies of Phase 6, which certainly equates with the current state of affairs, as does the building Scandinavian HIGH. This is not a cool phase for the UK.

MJO phase 6 positive ENSO

MJO Phase 7 in January

MJO Phase 8 positive ENSO

Phase 7 sees a HIGH pressure building out from Scandinavia as a possible northern block. This equates with the emerging negative NAO and potential easterly / Arctic winds winding round the base of the HIGH. It is an increasingly cool phase for Europe, so matches expectations as we move into early January. Phase 8 is a high pressure phase in Europe as the LOW moves further into the Mid-Atlantic. High pressure can be cool dry frosty at the surface depending on the exact location of the high pressure. This is expected by 13 Jan.

MJO Phase 1 +ENSO in January

MJO Phase 2 in Jan ENSO +

Jan MJO Phase 1 +ENSO surface temp

Jan MJO Phase 2 +ENSO surface temp

Finally, Phase 1 and 2 (above) are both cool phases as they build Atlantic / Greenland blocks to the NW and place a trough over the UK dragging in potentially cold NE or NW. These MJO phases could weaken according to MJO forecasts but the signal is due later Jan/early Feb. The MJO is just one teleconnection in winter weather forecasting it will be one to watch in the coming weeks and most interesting to see how it verifies with prevailing conditions. Below are some links to explore the MJO yourself.

This is a round-up of the atmospheric situation and not a forecast. In summary, however:

models are struggling with all the action, ecm might be preferred as gfs does not take account of much vertical extent into the stratosphere, while ecm does. ecm is showing colder runs generally.

stratospheric vortex is taking a hammering and a SSW is predicted for January (AER)

Very warm conditions have dominated this December, especially in SE England due to a persistent warm SW sub-tropical airmass. Here in Reigate the mean monthly December temperature so far is over 10C, nearly 5C above the longer term average for the month (5.3C). Records for daily maximum and highest minimum temperatures have been tumbling as 850hPa airmass temperatures have risen 5-10C warmer than normal, making it feel more like May or even June than December, especially overnight! In addition, the air arriving tonight is loaded with Saharan dust.

It is the warmest start to December for years but will it break the all-time absolute Tmax record of 18.3C?

all time December record Tmax

Tmax records for certain days in Dec

Oddly enough probably not, because the December absolute Tmax record was set in the lee of mountains in Scotland where, in the right conditions, a special foehn effect can lift temperatures beyond the reach of even the warmest airmass that we are likely to get in the next few days. This airmass is cloudy too, so the sun is less likely to break through.

Nevertheless, daily and local records here in the SE are tumbling and the duration of this warm spell is unusual. The cause is a stubborn high pressure over Europe and a trough in the Atlantic feeding a persistent SW airflow from the warm sub-tropics, places like the Azores.

Azores: nice!

Euro-high doggedly hanging on Dec 2015

warm December anomaly

At this time of year the Saharan HIGH pressure is pretty strong and the southern edge of this creates winds that pour across the Sahara Desert from the east, heading across the continent to the Atlantic.

Bodele depression funnels wind

high winds over Sahara

The Bodélé Depression in Chad is very likely to be the source of any dust arriving in the UK over the next few days. Weather stations nearby have recorded windy conditions almost entirely from the east.

Saharan weather Dec 2015

Sahara weather station 2015 Dec

The Bodélé Depression produces more than half of all Saharan dust, partly due to the way super-geostrophic wind circulate around the Saharan high pressure and are funneled through a mountain barrier into the depression, accelerating wind which then lofts prodigious amounts of dust into the air.

21 Nov 2015 largely sand-free Bodélé Depression

1 Dec 2015 dust storm in Bodélé Depression

Some 700,000 tonnes of dust can be lofted into the air every day in this location. The result is some extraordinary rock formations due to the eroding effect of the sand blasting the rocks.

Bodélé Depression strange rock formations

Bodélé Depression sand blasted

Bodélé Depression sand erosion

Desert sand tends to be lofted into the lower atmosphere, up to the boundary layer at most some 1km-2km or so above the surface but it can reach higher altitudes in some conditions and be transported further afield. From Chad it is blown by trade winds into the Atlantic. A lot of this dust continues across to America but, depending on conditions, some of it can be gathered into the mid-latitude circulation and make its way to Europe. Spot the journey of the dust on these sat pics. taken since late November.

Bodélé Depression

21 November 2015 largely dust free Sahara

dust storm early Dec

Desert dust arrives Atlantic Ocean

By Thursday the desert dust is forecast to join more local particulate pollution in a warm sector to bump up pollution levels in the UK. It also coincides with exceptionally mild airmass. A breeze tomorrow should reduce the threat of a “toxic cloud” developing as stated in some media. Nevertheless, it is worth considering that desert dust is an entirely natural and vital part of the atmospheric circulation.

17 Dec 2015 warm and dusty event UK

aerosol optical depth 17 Dec 2015

air pollution forecast UK Dec 17 2015

The outlook is for continued warm/mild weather to continue into Christmas, potentially more unsettled at times as the jetstream perks up and takes on a more direct zonal attack across the Atlantic. The bigger reason for the mild mid-latitude weather is an exceptionally powerful polar vortex that is keeping pressure and temperature very low in the polar stratosphere.

The lower tropospheric jetstream is subsequently strong and “locks in” the cold to the polar regions. This situation is summarized by a strongly positive North Atlantic Oscillation: indicated by high pressure over the Azores and low over Iceland. Until this situation changes the chances of sustained cold for us are slim. The only hope for sustained cold this winter in a mega-El Nino year is said by experts in long range forecasting, to be a sudden stratospheric warming that will break down the polar vortex. That can occurs most commonly Jan-Feb. Here’s hoping!

A cold Arctic surge will spill south over Friday night bringing a shock to the record-breaking warm November so far. The airmass trajectory shows the extraordinary journey the air has taken to get to London.

airmass trajectory (GFS)

It’s a brief Arctic attack but rather potent and Saturday morning will feel especially cold in strong northerly winds.

cool northerly / NE winds

strong winds and gales on coast

Snow is also forecast for Scotland and parts of the North of England as the Arctic air is brought south with a North Sea low and attendant fronts. This system will reach Reigate and the SE early Saturday morning and likely bring a wintry miserable start to the weekend with sleety rain, maybe even some wet snow for the Downs. (in the event: snowfall in Reigate for an hour or so as front passed through, settled momentarily. front cleared off quickly)

northerly jet from ice sheet to high street

Arctic plunge for November

In the South East, whilst the airmass temperature is just cold enough for snow and sleet, falling to a cool -5C at 850hPa by early Saturday morning, the dew point is shown to rise quickly above freezing as precipitation arrives in the morning and this will reduce the chance of any proper snow fall. This is fairly typical of the SE!

snow for north and east

turns to sleet/rain in SE

The main feature of Saturday morning in Reigate will be a truly miserable wet, cold windy start with an impressive wind chill well below freezing lasting all day, something we haven’t experienced in Reigate and Surrey for a long time!

wind chill November 21 2015

Overall, this cold snap is not due to last long, warming slowly into next week but turning unsettled as the jetstream blasts back from the Atlantic. It will not, however, become as warm as it has been.

unsettled due to return as jet bites back

Winter forecasters look high into the atmosphere to get an idea of what the winter will bring. One feature they look at is the Polar Vortex. The PV is currently getting stronger, which it usually does this time of year. The vortex is the winter circulation of high altitude westerly winds in the stratosphere that keep the cold polar air locked up in the Arctic. To get sustained cold outbreaks the vortex needs to be broken down: something that can occur when the polar stratosphere warms, sometimes suddenly.

strong polar vortex

North Pole: cold temperature

However, with North Pole stratospheric temperatures taking a steep dive, lower than average, the temperature gradient between the Pole and mid-latitudes is increasing and so the subsequent pressure gradient is also steep: with generally low pressure over the Pole and high pressure further south. This all really means cold air is less likely to leak south.

The Arctic Oscillation is an index showing how atmospheric pressure varies between the North Pole and mid-latitudes. In a positive phase the westerly winds are stronger and the jetstream more powerfully moves depressions across the Atlantic to the UK. A negative AO is required for wintry outbreaks… El Nino can induce stratospheric warming episodes and many winter forecasters are expecting the Polar Vortex to come under attack later this winter, after Christmas, and possibly produce a colder second half. meanwhile, expect more unsettled, relatively mild weather to return after this cold snap.

The brief cold snap is shown in the ensemble forecast from the GFS and ECM models below: cold snap followed by a return to mild.

September in Reigate turned out to be cooler than average with rainfall amounting to around average totals mostly falling in showers so that there were relatively long dry sunny spells, especially towards the start of the month, with HIGH pressure in charge or nearby for much of the time.

Reigate sunshine

Reigate pressure and wind

mean 500mb anomaly for September 2015

It was sunnier than usual for September at over 130 hours. This was good news for Run Reigate which turned out to be a beautiful day for the runners.

September anomalously sunny

Sunny Reigate sept 2015

unbroken sunny days occasionally

nice sunsets in Sept

Much of the September rainfall fell in heavy showers, especially in some storms mid-month. This meant 2015 September rainfall at nearly 79.8mm turned out wetter than 2014, at a mere 22mm. Despite this it was still just about equal to the long-term average rainfall according to MetOffice anomaly charts.

Reigate average rain

rainbow and showers over Reigate

Cb action over Reigate

September about average rainfall in SE England

At a mean temperature of 13.4C September was slightly cooler than average and the CET came out at 1C below the long term average. Globally, September was 0.31C warmer than the long term average.

Reigate experienced an impressive deluge today when over 60mm* (tbc) of rain fell in a few hours causing flooding in parts of the town. This was a “surface water flood” caused by drains being overwhelmed by intense heavy rainfall, rather than a river flood, our local River Mole will react more slowly to this event and is unlikely to cause any problems. (*by 4.30pm manual rain gauge 64mm; radar netweather est 50mm; total 24 hour rainfall 74mm manual rain gauge; RGS AWS 36.6mm; Reigate AWS 50.8mm)

The entrance to Morrisons car park and the lower section of Bell Street were overwhelmed with water bursting from drain covers adding to the water running down the roads. It wasn’t just the roads that were flooded… such was the intensity of rain that Priory Park was also left with surface water pooling up several inches deep on the pitches, and even more in the sunken garden and around the cafe.

Reigate Priory flooded

car parks flooded

Priory Park flood

sunken garden sunk

Priory flood Reigate

This rain event was accurately forecast by the MetOffice and MeteoGroup and several specialist severe and convective weather met agencies who all issued warnings of heavy thundery rain, some totals suggested over 60-90mm.

MetOffice warning

ConvectiveWX slight risk

MeteoGroup Orange warning

Estofex warning

Two models, the MetOffice Euro4 and WRF NMM model output, turned out to be especially accurate with both putting exceptionally heavy rain precisely over our area at least 24 hours in advance and this was reported by RGSweather on twitter. The GFS model was less convinced about such heavy rain reaching beyond the south coast but, as always, these convective summer events are especially hit and miss. Today was a “hit” for Reigate but unfortunately would have probably caused a nuisance for businesses and commuters.

The maximum intensity of the rainfall in Reigate was measured at 145mm/hr during the storm between 2-3.00pm. This intensity of rain is unusual in Surrey (highest rainfall intensity in recent years was 183mm/hr 20 Nov 2013) but we are told to expect more of our rain to fall in intense events like this with global warming. The last time flooding of this scale took place in town was 24 Dec 2014 but, having seen both events, I think this time was slightly “worse”, although given summer conditions the surface water may have subsided quicker. There are many areas for further investigation worthwhile here: how much rain, for how long and with what antecedent conditions (soil moisture / season / evaporation etc) and what duration of rainfall intensity causes flooding in Reigate?

rainfall intesity Reigate

The synoptic weather situation that caused this event was a trough over the Atlantic and a blocking HIGH over Scandinavia causing a warm unstable humid airmass to meet cooler Atlantic air along active fronts. The resulting LOW transported a lot of moisture over the SE and caused the rain in heavy thundery downpours triggered near the fronts. This situation is not likely to change much until later this week, so expect more rain but hopefully not as intense.

August SE Reigate flood

An unstable airmass with backing winds over the SE indicated by a skew t chart from the day.

skew t chart 24/08/15

backing winds over SE

trough disruption #1

trough disruption #2

Technically speaking this was part of a “trough disruption” event (see charts above) that started over the Atlantic during the weekend. A “trough disruption” is when an upper trough “breaks” and a southerly section of the trough proceeds purposefully east or NE leaving the centre of the trough behind as a semi-static feature. The isolated part of the trough can then behave erratically. In this case an unusually active and southerly dipping jetstream for the time of year also played a part in deepening the LOW at the base of the disrupted trough. Weather events can be “severe” and unexpected with models sometimes struggling to cope when trough disruptions occur.

Biscay LOW

rapid fall in pressure

dry intrusion kicks off severe weather

satpic from Monday shows LOW swirl

radar storm signature

satpic shoing 2 mature occluding LOWS

The LOW pressure that brought heavy showers over the SE and Reigate formed in Biscay yesterday and travelled NE up the English Channel during the day. SE England therefore experienced SE / easterly winds in the morning, backing northerly and finally swinging westerly during the course of the day, fairly unusual for our part of the world. The anticlockwise change of direction in wind is called “backing”. Veering is the clockwise movement of wind over time or with height.

RGSweather photos featured on the BBC London news that evening (sorry, no sound)

Finally, we can expect more rain this week as the synoptic situation stays static with the Atlantic trough blocked by the high out East. This situation will keep us in a flow of humid moist air that, when it interacts with cooler Atlantic airmasses along fronts, is likely to cause rain. Add an unusually active jetstream and we have a decent recipe for a wet week.

June in Reigate, Surrey continued the Summer 2015 theme of mostly cool, dry and sunny but with an unsettled start and THREE attempts at continental heat from Spanish Plumes in one month. (please note that the chart above hugely exaggerates the rainfall.. check the rain scale in mm. I have yet to fix down the scales month on month!).

Average Temperature 15.7C

Tmax 30.6C (25.6)

Tmin 5.5C (6.2)

Total rainfall 15mm (30)

sunshine 192 hours (175)

max gust 28mph

(Figures in brackets are from June 2014)

Reigate pressure and wind June 2015

Reigate sunshine hours June 2015

The month started unsettled with a deep low pressure 976mb crossing Scotland from a very much cooler-than-usual North Atlantic bringing brisk winds for the time of year and comparatively cool temperatures. Another Atlantic LOW crossed Scotland through the first week 991mb (see satpic).

991mb Atlantic occulsion

Occluded front 6 June 2015

6 June perfect occlusion close up

COOL Atlantic promotes HIGHS

cool North Atlantic anomaly

A weak first attempt at a Spanish Plume 5-6 June developed ahead of an Atlantic cold front and gave some limited thundery activity early morning on 5 June, these cells went on to become more significant further north east over E Anglia.

thunder 5 June

thunder 5 June reigate

sunrise thunder reigate

cumulus congestus

Pressure rose thereafter as an anticyclone built firmly over the UK. This HIGH eventually slipped north and a heat LOW from Iberia brought the threat of a second Spanish Plume around 12-13 June. This misfired and caused little convective activity over Reigate at least. This turned out to be a significant mis-fire for convective forecasters, despite some limited activity here and there the overall level of activity was low and certainly nothing occured over Reigate except very late in the day when some congestus puffed up.

Spanish Plume mis-fired 12 June 2015

High pressure built again with weak fronts skirting across the SE bringing some pleasant mid-level and upper level cloud, nice sunrise and sunsets and some good atmospheric optical phenomenon at times.

cirrus waves 18 June

halo Reigate 18 June

25 June Reigate sunrise with altocumulus

altcumulus castellanus sunset

Towards the end of June a third attempt at a Spanish Plume yielded more heat and more purposeful thundery activity that eventually spilled over into decent thunderstorms into the start of July. This was a modified Spanish Plume and more details can be found on the post written up here and here.

altocumulus castellanus

plume III lightning

The end of June 2015 heat spike produced some 30C+ temperatures and in Reigate 30.6C was recorded on 30 June. Overall the month was sunny but a shade cooler than average according to the CET central england temperature record.

temperature anomaly june 2015

geopotential height anomaly

June 2015 tempesunshine hoursrature anomaly

The UK anomaly charts reflect the overall dry pattern with notably little rainfall for the month. In Reigate the total rainfall measured was 15mm.

dry UK

june 2015 precipitation anomaly

South East England rainfall June 2015

2015 rainfall

Although this June was not consistently hot tp push up the CET (central england temp), we did nevertheless have episodes of unusual heat, especially at the end of the month with the end of June / early July heat spike. This heat wave was more severe and prolonged in Europe. A “cause” of the Euro heat wave, with significant heat in Spain and Portugal, was an OMEGA BLOCK or “shruggie” that built through June and lasted into early July. This Omega Block pattern may also be linked to rapid melting of the Greenland icecap that has been recently reported as heat builds there under high surface pressure.

A weather pattern that resembles an atmospheric version of the shruggie — ¯\_(ツ)_/¯ — is directing furnace-like heat toward Spain, France and England on Wednesday, with high temperatures near 40 degrees Celsius, or 104 degrees Fahrenheit, as far north as Paris.

The heat wave is also affecting southern parts of England, with temperatures in the upper 30s Celsius, or mid-to-high 90s Fahrenheit. Those temperatures on Wednesday were the warmest recorded in the UK in at least nine years, according to the UK Met Office.

June has been a cool month so far and is on target to be the coolest June since 1991, or certainly close. It has also been dry with just 28% of monthly rain so far for SE England.

cool June

and dry..so far

Azores HIGH and keeping us mostly dry

In contrast to the June trend, mostly due to the Azores HIGH lodged firmly to the SW and dragging in a cool dry NW flow (bit unusual for a summer regime), today a humid moist warm sector crosses Reigate and this will bring a minor hiccup to the dry weather of the last week when there has been no rain at all in Reigate. Humid air is due to arrive in the warm sector and this is indicated by modestly raised dew points (high teens) and PWAT (precipitable water) exceeding 30mm, showing potential for some heavy rain around but this is hit n miss. (update: metoffice weather warning issued in morning for heavy thundery rain)

synoptic scene: warm sector

dew point

metoffice wx warning

sparking storms by Sat pm (real)

Of interest are surface winds which appear to converge in a zone, associated with a cold front, across the south later today (spot the twisty wind barbs below): convergence is where winds arrive quicker than they leave a region and, as winds “pile up”, this often promotes LIFT (upward air motion) that encourages convection and storms. But it’s a slack scene and by no means a classic stormy picture. Scattered slight risk of heavy showers is likely to be about it.

converging winds

Skew-t charts (cross-sections through the atmosphere showing temperature, wind speed, direction and humidity etc) also show reasonably high CAPE >600j/kg (convective available potential energy) and some negative lifted index in central southern England. These values are indicators of instability: which means air is free to rise to a great height, condense into tall clouds possibly forming cumulonimbus. Note the change of wind speed with height, such wind shear also acts to duct air from the surface. Well, the ingredients are there for heavy showers later today but they were also present in the much heralded plume last week and that came to nothing, catching out professional forecasts as well as amateur enthusiasts. There were a few notable heavy thundery outbreaks last week but many convective forecasters and storm enthusiasts were stung by the lack of activity and model predictions appeared to founder. The scenario today is decidedly less “stormy” so storm fans should not get excited either!

Of note is a dry slot at 700hPa mid levels (shown well on the chart below) that can induce evaporative cooling. Evaporative cooling is caused when moisture evaporates and reduces temperature. This reduced temperature at mid levels of the atmosphere can create higher CAPE as warm parcels excitedly find themselves rising through ever cooler environmental air. If sunshine heats the surface this can promote heavy showers and possible thunderstorms, albeit scattered and only a slight risk today. In addition a weak jetstream moves to a position later pm where any showers will find themselves on the left-exit region of the jetstream.. this is area known to further lift air from the surface, like a hoover dragging air upwards into divergent air aloft.

high cape low LI

trough, diffulent

dry slot

slight tstorm risk

left exit of jetstream

The forecast is for scattered showers, some thundery later. So, as usual, some places could miss them though it’s a moist air flow so some rain is likely most places. Rainfall totals will vary between almost nothing to possible >10mm.

more unsettled to end June

Sunday looks cloudy but mostly dry as showers clear off tonight. Next week and towards the end of June there is a threat of more rain, possibly pretty heavy on Monday as Atlantic LOWS nibble away at the Azores high that has dominated our weather recently. Monday sees a frontal wave low sticking wet conditions across the south which could yield high rain totals. Thereafter, mid week sees the Azores HIGH ridging back in with pleasant warm and dry conditions but this looks temporary as Atlantic LOWS nibble away with wetter westerlies always trying to edge back in.

Azores ridges back in, but looks temporary

The outlook is therefore occasionally unsettled, especially in the north further from any ridges, though with the risk of heavy showers at times in the south. Gradual warming trend into July with possibility of a brief warm or hot SE flow to start the month, as shown below on the 850hPa temperature GEFS chart: note the “plume” (oh no) with regressed Azores HIGH and thermal heat LOW over Spain. Way off, but worth watching 🙂

April in Reigate was a mostly dry, very sunny, rather calm and relatively warm month. However, the settled weather came at a price with a significant air pollution event on 10 April. Also, April showers were conspicuous by their almost complete absence so convective weather fans were left disappointed.

Here are some April details for Reigate:

Tmax 14.4C

Tmin -1.9C

Average Temperature 6.8C

Total rainfall 23mm

Max gust 46mph

sunshine total 136 hours (sunniest April on MetOffice records!)

UK dry April

47% of usual rainfall

April 2015 dry up Europe

April was 43% drier than average across SE England and this shows up in the diminutive 23mm of rain in Reigate. Pressure rose early in the month and so April turned out 150% sunnier and about 1C warmer than average too. The central England temperature for April came out at 1.1C above long term average.

It was the SUNNIEST April on record for the UK according to the MetOffice. In Reigate we enjoyed 136 hours of sunshine in April.

sunny April 2015

sunnier than average

pressure: red and blue; max gust green line

April warmer than average

Europe warm in April

The significant downside of this calm weather was high AIR POLLUTION. There was a high pollution warning for a time when easterly / south easterly winds brought high levels of PM2 particles into the SE and across London from Benelux countries.

SE pollution warning

S/SE winds bring pollution

air pollution maps

air mass back trajectories UK

pollution levels

yellow air pollution sunrise Reigate

Here are some photos from Reigate during April showing what a mostly very pleasant month it was.

High pressure bringing fine sunny weather to much of Britain is shifting over the continent and set to bring a brief high or very high pollution event across the SE of the country on Friday due to a subtle shift in wind direction. Slack air residing over Benelux countries for several days is set to move our way.

current pollution levels

polluted air from Europe

Belgium, Luxembourg, Netherlands and Northern France are populated, urbanised and industrial areas of Europe and the longer an air mass sits over such places the more pollutants it picks up. The air over Benelux has been sitting slackly over these locations collecting pollution for several days (see back-trajectory chart below). This air is forecast to travel over to the SE of the UK through Thursday bringing the highest levels of pollution to our area by on Friday morning.

Friday can be a particularly bad day for pollution events because of the build-up of local pollution through the week (which has also previously been correlated with greater rainfall on Fridays due to additional condensation nucleii being present in the atmosphere). This local pollution will add to the toxic mix of imported long range air pollution expected to arrive here on Friday morning. Fortunately it is due to be replaced fairly quickly by fresher cleaner westerly Atlantic air by Saturday.

back-trajectory shows origin of polluted air

temperature inversion puts lid on pollution

The situation is worse on Friday because of a temperature inversion at low levels persisting during the morning (see above right). Temperature inversions often occur in high pressure situations overnight when air near the surface cools by radiation. The cooler air near the ground is unable to mix vertically because it is trapped by a warmer layer of air aloft, this is especially common on cool mornings and can be observed when mist or fog lingers at the surface. It is such trapped surface air that can build up most polluted conditions and causes most health impacts. In the most persistent inversions this can cause a condition known as “fumigation” (see below). In addition, at higher atmospheric levels a plume of dust whipped up from the Sahara is likely to increase any haze observed.

forecast pollution levels Friday am

dust blowing from Sahara

inversion fumigation

Air pollution can be produced locally by vehicle exhaust pollution, industry and agricultural activities (lofting of slurry products and ammonium based fertilizers for example). Long range air pollution transport is imported from elsewhere.. non-dom pollution! Ozone pollution at the surface is not produced directly but is the product of a chemical reaction with polluting gases such as Nitrous Oxides and strong sunlight. This creates ozone gas which, at the surface, is a pollutant. Fortunately, ozone levels are not expected to be high on Friday but particulate matter certainly is.

friday pollution forecast

levels of pollution

pollutant base line

Fortunately this is a brief pollution event because the wind is due to shift quickly to the SW/W through Friday which will introduce fresher, cleaner Atlantic air by Saturday after the passage of a cold front.

cooler fresher air by Saturday

A bit more detail on air pollution: Air pollution is a mix of tiny particles and gases such as sulphur dioxide, nitrous oxides and ozone. Particulate matter has sources that can be natural, such as volcanoes, dust from deserts, fire, spores and pollen. Particles from natural sources are usually bigger in size and are less damaging to human health than smaller human-made / anthropogenic particles from the burning of fossil fuels (such as coal), other chemical reactions and agricultural activities. Exposure to these smaller PM2 or PM2.5 particles can be more hazardous to human health because they can contain toxic heavy metals, can be soluble and, due to their tiny size, can be breathed deeper into the lungs and enter the cardio-vascular system causing health risks such as lung cancer, asthma and even heart attacks. There is “no safe threshold” identified by DEFRA for these smallest of deadly particles PM2 so any exposure is worth avoiding.

PM2 particulate impacts

levels of pollution

DEFRA advice on how to respond to pollution events is listed below. Importantly, avoid unnecessary exercise, breathe through your nose if possible and stay indoors if you are in a vulnerable group (children, elderly and people with lung problems). In poor air quality conditions you might notice your mucus build up more than usual and perhaps coughing too as a response.

Exposure risks to air pollution

The outlook for Reigate is for the weekend to be a little more unsettled and cooler as a cold front passing through on Saturday, but little rain is expected to reach us in the SE. Sunday looks dry too but rain further north might slip south so watch this one. Then a return to high pressure into next week and more dry conditions for the first half of the week. It looks like the second half of April could turn more unsettled, but nothing dramatic is on the cards locally.